Can vegetation types work as an indicator of soil organic carbon? An insight from native vegetations in Nepal

Soil is the largest pool of carbon after the ocean. The assessment of soil organic carbon (SOC) stocks under different types of vegetations is critical for evidence-based implementation of soil carbon trading under different market mechanisms. Using data from 490 permanent soil sample plots with elevations ranging from 271 m to 3238 m above sea level, from three different watersheds in Nepal, this study aims to compare SOC under five different forest types and determine whether forest types could be indicator of SOC: Shorea robusta; mixed broadleaf; Schima-Castanopsis; pine; and Rhododendron-Quercus forests. In each vegetation type forests with dense canopies (≥70% canopy cover) have higher amounts of SOC than that of open canopy forests (<70% canopy cover). On average, with up to 30 cm soil depth, dense canopy Rhododendron-Quercus forest has the highest amount of SOC (14,136 g C/m2), followed by dense canopy mixed broadleaf forest (12,576 g C/m2). One-way analysis of variance (ANOVA) indicates significant differences in soil organic carbon (SOC) amounts across the five forest types (df = 485, F = 17.299, p = 0.000) and therefore forest types could be indicator of SOC. Moreover, within the similar altitudinal zone and topographic, edaphic and climatic environments: soils under the mixed species forests have higher amounts of SOC than that of single species dominated forests; and soils under forests with nitrogen fixing trees have a higher amount of SOC than those from other forests. Therefore, in addition to forest types these two conditions within the given vegetation types could work as sub-indicators of SOC. In comparison with global studies, all Nepalese forest types had much higher levels of SOC. Only two of the seven factors/indicators investigated (altitude and average age of dominant trees) were found to have significant impacts (r = 0.33 to 0.64) on SOC and only in the dense and sparse canopy pine forests. The reasons of not having any impacts of all other factors on SOC in different vegetation types have been discussed.